There are a number of optical metrology systems and applications including scatterometry, polarimetry, spectroscopy and ellipsometry that require a broadband test beam. The large bandwidth or wavelength range Δλ of the radiation used in these test beams imposes a set of constraints on the optical system and on a number of its operating parameters. The most important constraints are the increasing wave front distortion encountered when employing refractive optics in apparatus for these applications as well as chromatic dispersion.
To limit chromatic dispersion, a number of prior art references teach the use of all-reflective optics, i.e., various types of reflectors or mirrors. For example, U.S. Pat. Nos. 5,991,022 and 6,128,085 to Buermann et al. teach the use of specific types of mirrors, namely toroidal ones in a spectroscopic apparatus. The use of toroidal mirrors for reflectance measurements in particular is also taught by Mandella et al. in U.S. Pat. No. 6,075,612. U.S. Pat. No. 6,181,427 to Yarussi et al. teaches a compact optical reflectometer system that employs flat mirrors and U.S. Pat. Appl. 2004/0047053 to Li teaches the use of ellipsoidal reflectors for coupling light from a source to a target. U.S. Pat. No. 6,611,330 to Lee et al. illustrates the use of a mirror system for performing polarimetric measurements that may be combined with ellipsometry using a beam of broadband radiation.
In addition to managing chromatic dispersion, optical systems frequently require spatial filtering in order to ensure that only radiation coming from a certain place is detected or imaged by the system. The most well-known approach to spatial filtering involves the use of apertures or pinholes. These are used extensively in fields such as confocal microscopy, e.g., as described in U.S. Reissue 32,660 to Lindow et al. and U.S. Pat. No. 6,870,609 to Watkins et al. Prior art optical metrology apparatus also employ spatial filtering, for example in the field of ellipsometery, as shown in U.S. Pat. No. 6,456,376 by Liphardt et al. Still other examples of spatial filtering in a metrology device that operates in reflectance mode, transmittance mode or mixed mode are found in U.S. Pat. No. 6,842,251 to Holden et al.
A more specific subset of optical metrology applications involves measuring samples with broadband radiation that is transmitted through the sample. Such measurements can be performed when the samples or substrates under study are transparent or semi-transparent. For general information about transmission metrology the reader is referred to U.S. Pat. No. 6,891,628 to Li et al. Still other optical systems that employ reflective optics for measuring samples such as wafers are described in U.S. Pat. No. 5,764,365 to Finarov and U.S. Pat. No. 6,734,967 to Piwonka-Corle et al.
Unfortunately, although some of the above prior art approaches manage to reduce chromatic dispersion, they are not capable of examining samples with small spots exhibiting low wave front distortion. Furthermore, the prior art systems are not able to combine the low wavelength distortion constraint with compensation for chromatic dispersion of light transmitted through the sample and contemporaneous and efficient beam monitoring. This is especially true in systems that perform transmittance measurements and in which the chromatic dispersion affects broadband radiation that is incident on the sample at off-normal angles.